1. Field of the Invention
This invention relates to the atomic detection art and more particularly to an improved method and apparatus for detecting the presence of very low atomic densities in a gaseous sample.
2. Description of the Prior Art
In many applications such as monitoring trace carcinogens and other environmental pollutants, measuring the constituents of combustion products, measuring biological tracers and chemical vapor tags as well as contaminants in industrial processing it is desirable to be able to detect the presence of comparatively very low densities, for example, on the order of a million down to 1 atom per cubic centimeter. Further, in detecting possible contamination from nuclear waste products the detection of such low concentration of atoms is necessary for establishing procedures to protect both personnel and the environment. Further, to provide ease of detection techniques, it is often desirable that such detection be accomplished in non-enclosed environments and, preferably, at atmospheric pressure.
One prior art method of detecting the presence of such atoms in the environment is atomic absorption spectroscopy. However, the range of detection limits for atomic density utilizing atomic absorption spectroscopy is on the order of 10.sup.8 to 10.sup.10 atoms per cubic centimeter
Another method heretofore utilized for detecting the presence of atoms is atomic fluorescence. In atomic fluorescence techniques, while detection limits have been, in certain specific cases, lowered to approximately 10.sup.2 atoms per cubic centimeter, they have generally required utilization of comparatively low pressure environments for containing a gaseous sample having the atoms to be detected. It has been found that, for example, at one atmosphere pressure Rayleigh and Mie scattering background emissions and quenching limit the practical use of atomic fluorescence detection techniques to such low pressure conditions.
Detection at pressures of approximately one atmosphere not only provides greater flexibility and more widespread utility in the application of atomic detection but also allows utilization of various atomization sources such as chemical flames, ovens, and electric furnaces to provide the necessary gaseous sample containing the atoms to be detected.
Thus, there is a need for atomic detection and quantitative measuring techniques that not only will allow detection limits down to atomic densities on the order of one atom per cubic centimeter but also for methods that can be used for samples in ambient pressures of one atmosphere.
In other applications such as, for example, quality control techniques in semiconductor fabrication, it is desirable to provide a micro area probe for scanning the surface area of a semiconductor to detect imperfections in, for example, a particular layer of a multi-layered structure. If such imperfections exist, atoms from a sub-surface layer may escape through the imperfection and detection of such escaping atoms down to very low atomic densities can indicate the presence of exceptionally small imperfections.